The invention relates to a cathode structure intended to be inserted into an electron gun for a cathode-ray tube.
The current trend is tending towards requiring cathode-ray tubes of increased performance in terms of screen brightness, lifetime, bright-up time and consumption.
Most of these parameters essentially depend on the structure and the type of cathode used to generate the electron beam or beams which will scan the screen of the tube.
The oxide-coated cathodes normally employed hitherto are at their limits with regard to these requirements, and the tendency is to replace them with dispenser cathodes which make it possible to achieve higher current densities with longer lifetimes.
Dispenser cathodes or impregnated cathodes operate at temperatures of about 1000xc2x0 C.-1200xc2x0 C. The expansions of the constituent materials of the cathode at these temperatures have to be minimized in order to obtain good performance stability of the electron gun into which this type of cathode is inserted, this being achieved by using a refractory material and dimensions of the cathode support which limit the conductive heat losses.
To achieve this result, Patents U.S. Pat. No. 4,184,100 and U.S. Pat. No. 5,218,263 illustrate two types of structure commonly employed:
a cathode body of substantially cylindrical shape having, at one end, the emissive part and containing the heating element;
a metal sheath which serves as shielding, by acting as a heat shield, of substantially cylindrical shape surrounding the cathode body; and
a means for supporting the cathode body inside the shielding.
The supporting means must be such that they allow rigid assembly while still minimizing the conductive heat losses. The means for supporting the cathode body may be tabs, produced from metal strips, of small cross-section in order to minimize the thermal losses, the ends of these tabs being connected on one side to the cathode body and on the other side to the shielding. In another embodiment, the tabs are cut out from the cylindrical part of the shielding so that one end remains integral with the shielding while the other end is connected to the cathode body.
However, the fact of using metal tabs of very small cross-section does not enable sufficient mechanical rigidity to be achieved, when fitting the cathode body in its sheath and fixing it thereto; moreover, these connection tabs may be easily deformed by the thermal expansion of the cathode body during its operation.
Patent Application EP 534,842 describes a structure in which the supporting means are more rigid, since they consist of a substantially frusto conical ring whose surface is pierced with holes. However, this structure has the drawback of promoting heat exchange between the cathode body and its sheath, thereby impairing the thermal efficiency of the cathode.
An object of the present invention is to improve the cathode support structures in order to allow both rigid assembly of the cathode in its support and minimization of the conductive heat losses between the cathode body and its sheath.
To do this, according to one embodiment of the invention, the cathode structure comprises means for supporting a cathode body inside a metal sheath, wherein the supporting means include a first series of branches connected to the metal sheath via one of their ends and a second series of branches connected to the cathode body via one of their ends, the second ends of the first and second series of branches being connected together by an intermediate piece.